Flow Battery Technologies for Utility-Scale Systems

The use of grid-tied energy storage system (ESS) batteries is growing fast. Large high-power batteries are also being used for so-called micro-grid applications, supplying power for lighting and air conditioning in remote areas. If you want to live off-the-grid using solar panels, you’ll need energy storage.

41.8MW of energy-storage type batteries were deployed across the U.S. in the third quarter of 2017. This is a 46% year-over-year growth and a 10% growth over the previous quarter. Reports say that lithium-ion chemistry was 97% of all energy-storage capacity deployed in 2016.

Non-Li-ion battery technology researchers and start-up companies are hoping to take a piece of the rapidly growing utility-scale battery storage industry. There are many, many new battery types being developed for energy storage. A new one every day, it seems.

A couple stand out to me. Both are flow batteries, a type of rechargeable battery where the battery stacks circulate two chemical components dissolved in liquid electrolyte. The two electrolytes are separated by a membrane within the stack, and ion exchange across this membrane creates the current flow while both liquids circulate in their own respective spaces. Flow batteries offer the huge benefit of nearly infinite charge/recharge cycles.

Flow Battery at MIT and Ambris

Researchers at MIT have developed a new molten-electrode energy storage battery. This battery for grid-scale installations is based on electrodes made of liquid metals and is fitted with a new type of metal mesh membrane. The liquid metal contained by the mesh can be sodium, but it doesn't have to be. The other liquid metal of the cell can be lead, tin, or zinc. This is a major departure from the sodium-nickel chloride battery which relied on solid nickel and solid nickel chloride. The molten salt is composed of chlorides of the metals in the electrodes.

"I consider this a breakthrough," Professor Donald Sadoway of MIT says, "because for the first time in five decades, this type of battery—whose advantages include cheap, abundant raw materials, very safe operational characteristics, and an ability to go through many charge-discharge cycles without degradation—could finally become practical."

While some companies have been providing liquid-sodium batteries for specialized uses, "the cost was kept high because of the fragility of the ceramic membranes," said Sadoway. "Nobody's really been able to make that process work." This includes GE, which spent nearly 10 years working on the technology before abandoning the project.

After experimenting with various compounds, the MIT team found that a steel mesh coated with a solution of titanium nitride could perform all the functions of the previously used ceramic membranes, but without the brittleness and fragility.

These cells operate at elevated temperature and, upon melting, the three layers self-segregate and float on top of one another due to their different densities and levels of immiscibility. While it may seem crazy to use 'molten' components, it really isn’t. The cells are 80% efficient, with that 20% inefficiency given off as heat. And that heat is held within the system's insulated box and is sufficient to keep it at operating temperature. You just charge and discharge to keep it at that state.

In 2010, prior to the current battery development, Sadoway and David Bradwell co-founded a company called Ambri, with the goal of commercializing the flow battery technology. The base unit for Ambri’s system is a fully sealed liquid metal battery cell (Figure 1). The company’s cells are strung together within a thermal enclosure to form an Ambri Core. This is insulated and "self-heating" when operated every couple of days, requiring no external heating to keep the batteries at operating temperature.

Other Flow Batteries

A wide range of chemistries have been tried for flow batteries. There are at least 15 different make-ups currently in R&D or production. Traditional flow battery chemistries have both low specific energy (which makes them too heavy for electric vehicles) and low specific power (which makes them too expensive for stationary energy storage). However, a power of 1.4 W/cm2 was demonstrated for hydrogen-bromine flow batteries, and a specific energy (530 Wh/kg at the tank level) has been shown. Again, they offer the huge benefit of nearly infinite charge/recharge cycles.

ViZn Energy is producing flow battery systems in sizes from 3kWh to 960kWh that feature high reliability and 20-year lifetimes. Based in Austin, Texas, and Columbia Falls, Montana, they have been working on battery technology since 2009. Their battery (Figure 3) is a hybrid flow type in which alkaline electrochemical components are dissolved in the electrolyte. They run on a safe chemistry that is non-toxic, non-flammable, and non-explosive. In fact, the electrolytes used are food-grade and found in abundance at commodity prices. ViZn tells customers to feel free to cycle its battery at full power from 100% state-of-charge to 0% and back to 100% again multiple times a day. They say they have achieved more than one million cycles without any capacity changes or impacts on basic performance.

At Intersolar North America in July of 2017, the company said that energy storage could now be added to grid-scale wind or solar PV installations at a "record low price" of just US$0.04 per kWh for a 30MW, four-hour duration system. Bloomberg New Energy Finance has benchmarked this at around US$0.06 per kWh.

UniEnergy Technologies, based in Mukilteo, Washington, produces a vanadium flow battery (VRB)—­the Uni.System™ (Figure 4). This system delivers 600kW of power and 2.2MWh maximum energy in five 20' modular containers - four battery containers and one inverter and transformer container. VRB systems do not have the same fire or deflagration risk as Li-ion-based energy storage systems. While not flammable, the electrolyte in VRB systems is corrosive. It is comprised of a sulfuric-acid based solution—15% vanadium, 25% sulfuric acid, 60% water.

The battery energy storage system is the enabling technology for the modern electric grid. These systems improve grid reliability and support the ability of grid operators to manage increasingly variable loads and resources like wind and solar. In addition, microgrid systems using solar and wind with ESS can provide power in remote areas.

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